Computed radiography (CR) performance is tightly coupled to the overall image quality and detection capabilities of the entire image acquisition, processing and display chain. For diagnosis or during technical image quality testing patient—or target (phantom)—images are created on an intermediate storage medium, called image plate or detector. During exposure the image plate traps the locally impinging x-rays and stores the latent shadow image until it is scanned and converted into a digital image by a digitizer.
Physical process limitations and tolerances during image plate manufacturing generate local sensitivity variability across the detector surface. Storage phosphor based (amorphic or crystal) CR detectors are multi-layered structures composed of a substratum, an adhesion layer, a conversion and storage layer and a protective sealing layer. Each of these functional layers and their interfaces may suffer from various levels of typical imperfections, blemishes and artifacts causing locally deviating image plate sensitivity. The medium to high spatial frequency components of the relative sensitivity distribution across a detector's surface reflect the image plate structure (IPS), the detector's unique signature.
A CR-image should closely reflect the patient's or object's x-ray shadow information. Since the detector's local sensitivity is the multiplicative factor controlling the conversion of the latent dose information into the image signal, the IPS is inevitably water-marked into each CR image acquired from it. Local image plate sensitivity variability can by consequence lead to diagnostic image quality loss because the relevant patient information is polluted by the detector's IPS. Like dose-related quantum (photon) noise and digitizer noise, the IPS is a detector-related, disturbing noise source which diminishes the Detective Quantum Efficiency (DQE) of the CR system. Excessive IPS thus reduces the radiologist's reading comfort and confidence level since it becomes more difficult to discern subtle but important image information.